1. Field of the Invention
The invention relates generally to the field of nuclear magnetic resonance (“NMR”) analysis of chemical composition of fluids. More particularly, the invention relates to methods and apparatus for determining molecular weight distribution of hydrocarbons in crude oil using NMR measurements.
2. Background Art
Analysis of liquid phase hydrocarbons (“crude oil”) produced from subsurface reservoirs include molecular weight distribution analysis. Such analysis can assist the reservoir operator in determining appropriate extraction techniques to optimize the economic value of the reservoir and to reduce the possibility of premature failure of one or more wellbores used to extract fluids from the reservoir.
Molecular weight distribution analysis can be performed by many different techniques in the laboratory. Such techniques include, for example, liquid chromatography, gas chromatography, and NMR relaxation spectroscopy. NMR relaxation spectroscopy has the advantage has the advantage of not requiring and chemical change to the sample being analyzed. As a result, NMR relaxation spectroscopy is well suited for use both in the laboratory and at the location of the wellbores used to extract the hydrocarbons from the subsurface reservoirs.
A techniques for analyzing molecular weight distribution in crude oil samples is described is U.S. Pat. No. 6,859,032 issued to Heaton et. al. and assigned to the assignee of the present invention. The method disclosed in the '032 publication includes measuring NMR data of the mixture using an NMR tool or a laboratory NMR instrument, deriving at least one parameter for each observed constituent in the mixture from the NMR data, and calculating a molecular property for each observed constituent in the mixture from the at least one parameter. Methods according to some embodiments use correlations between relaxation times and molecular properties and/or between diffusion rates and molecular properties.
It has been observed that certain NMR properties, e.g., transverse relaxation time (T2), and self diffusion constant (D) of hydrocarbons may be related to their relative concentration in a mixture of hydrocarbons. See, R. Freedman, et al., Field Applications of a New Nuclear Magnetic Resonance Fluid Characterization Method SPE Reservoir Evaluation & Engineering, 455 (Dec. 2002). The investigation described in the foregoing publication related to mixtures of hexane and squalene and it is believed that the principle suggested therein is applicable to crude oil mixtures of hydrocarbons. What is needed is a method for analyzing mixtures of hydrocarbons using NMR measurements that is relatively insensitive to the relative concentrations of the constituent components.